Journal of the American Mosquito Control Association, 18(4):280-283, 2002
Copyright © 2002 by the American Mosquito Control Association, Inc.
THE STRUCTURE AND FUNCTION OF THE LARVAL SIPHON AND
SPIRACULAR APPARATUS OF COQUILLETTIDIA PERTURBANS
PETER J. BOSAK 1 and WAYNE J. CRANS 2
ABSTRACT. The structure of the larval siphon and spiracular apparatus of Coquillettidia perturbans and the
mechanism of attachment to roots of emergent aquatic macrophytes were examined by utilizing dissection and
scanning electron microscopy. The roots of these plants contain large air-filled aerenchyma channels that larvae
of Cq. perturbans pierce with their specialized siphon and spiracular apparatus to breathe. The siphon contains
the spiracular apparatus, comprising the saw, postabdominal spiracles, inner spiracular teeth, and the spiracular
apodeme. These are the primary structures that are utilized by larvae to pierce root tissue. Once entry is made
into a root, the outer spiracular teeth open fully, anchoring the larva in place.
KEY WORDS Coquillettidia perturbans, cattail mosquito, aerenchyma, siphon, emergent aquatic macrophyte
INTRODUCTION
Some of the most remarkable larval respiratory
adaptations in the family Culicidae occur in the
genera Mansonia and Coquillettidia. The species of
these genera have a specialized siphon to pierce the
roots, stems, or submerged leaves of aquatic plants,
enabling them to utilize oxygen from plant tissue.
One species, the cattail mosquito {Coquillettidia
perturbans (Walker)) is a common nuisance mos-
quito in North America that has been implicated as
a bridge vector of eastern equine encephalomyelitis
virus (Carter et al. 1981, Francy 1982, Sofield et
al. 1983, Crans and Schulze 1986, Nasci et al.
1993). Walker described the adult stage of this mos-
quito in 1 856, naming it Culex perturbans, but be-
cause of its unusual behavior the larva remained
undescribed for more than 50 years. The mystery
was solved in 1907 when J. Turner Brakeley, a vol-
unteer field observer in New Jersey, washed a 3rd-
stage larva from the roots of an aquatic grass grow-
ing in a marsh near Trenton. He reported the find
to John B. Smith of Rutgers University, who in turn
published a detailed account of the discovery
(Smith 1908). Smith's publication incorporated
John A. Grossbeck's description of the larva and
several drawings of its morphology, including that
of the siphon. After Brakeley's discovery, other
mosquito species that utilized some type of aquatic
plant as a means for larval respiration were discov-
ered in other parts of the world (Dyar and Knab
1910, Ingram and Macfie 1917, Wesenberg-Lund
1918, Edwards 1919, Gillett 1946, Laurence 1960,
Burton 1965). Over the years, researchers have ver-
ified many of Smith's initial observations regarding
Cq. perturbans; however, apparently no close ex-
amination was made of the structure and function
of the siphon and spiracular apparatus.
In this paper, the morphology of the siphon and
spiracular apparatus of Cq. perturbans are reex-
' Cape May County Mosquito Control Commission, PO
Box 66, Cape May Court House, NJ 08210-0931.
2 Mosquito Research and Control, Rutgers University,
180 Jones Avenue, New Brunswick, NJ 08901-8536.
amined and the mechanism for penetration and at-
tachment to roots of aquatic macrophytes are de-
scribed. Harbach and Knight (1980) are followed
for morphological terminology.
MATERIALS AND METHODS
Larval Cq. perturbans were collected at Colliers
Mills Wildlife Management Area, Colliers Mills,
NJ, by using the modified bilge pump method of
Walker and Crans (1986). In the laboratory, 4th-
stage larvae were isolated and placed in a 250-ml
beaker with 100 ml of bottled water. Fourth-stage
larvae were selected because this size enabled the
clearest view of the siphon and the attachment site.
Living roots of known host plants were cut from
masses in lengths of approximately 10-15 cm and
placed in beakers with several larvae. During the
warmer months, the setup was left at ambient tem-
perature, usually overnight, until a number of the
larvae attached to the roots. During the colder
months, the setup was placed in refrigeration at 6°C
overnight. Some larvae would not attach for vari-
ous reasons, and those that were going to attach
would usually do so overnight; allowing more time
did not result in significantly more larval attach-
ment and may have actually resulted in some de-
tachment. Once the larvae attached, the setup was
gradually frozen overnight. With this method, lar-
vae slowly freeze while remaining attached to the
roots. Larval Cq. perturbans are very cold tolerant
and may even withstand freezing for short periods,
so it is important to freeze the setup overnight.
Once thawed, small sections of root with attached
dead larvae were cut free with dissecting scissors,
carefully removed with forceps, and placed in a pe-
tri dish containing water. Free-hand cross-section-
ing of the root with attached larvae was performed
under a Leica StereoZoom 6 Photo dissecting mi-
croscope (Leica Microsystems Inc., Buffalo, NY)
at 40 X. After sectioning, the siphon and the at-
tached root cross-section were dissected from the
larva. The prepared sections were placed in a 10%
NaOH solution and incubated at 35°C overnight for
280
December 2002
Spiracular Apparatus of Coquillettidia
281
clearing and subsequently stained with a weak so-
lution of Safranine O.
For scanning electron microscopy, 4th-stage lar-
vae were collected as above and living specimens
were air-dried on filter paper. Photographs of struc-
tural details were taken with a Hitachi® S510 scan-
ning electron microscope (Hitachi Instruments, San
Jose, CA) after coating specimens with gold-pal-
ladium.
RESULTS
The siphon and spiracular apparatus (SAp) of
Cq. perturbans is a modified culicine type, and is
dark brown and strongly sclerotized. The siphon is
continuous, lacks sutures, and tapers gradually for
approximately one half of its length. The apex of
the siphon is abruptly constricted and bears the an-
teriorly curved spiracular apparatus that terminates
sharply. As with other mosquito larvae, Cq. per-
turbans rely on valves to open and close the SAp
as needed. When larvae are detached, either swim-
ming or resting, the valves remain closed and are
composed of several sclerotized movable plates that
are relatively smooth (Fig. la). The posterior aspect
is less complex than the anterior and consists of the
posterior spiracular plate and posterior spiracular
lobe (PSL), 2 structures involved in covering the
spiracular apparatus (Fig. lb). The anterior aspect
consists of a number of structural elements, the
most prominent of which is the saw. The sclero-
tized saw, situated within a furrow in the center of
the anterior portion of the siphon, is bordered ex-
ternally by the anterolateral spiracular lobes (LSL)
(Fig. lc). Internally the saw is fused basally to the
postabdominal spiracles (PAS) and these in turn are
joined to the spiracular apodeme (SAd). The PAS
are 2 fused, rigid tubes that connect with the large
flexible tracheal trunks in the 8th abdominal seg-
ment. The SAd is a laterally compressed tubular
structure that has a strong muscular attachment at
its proximal end and distally forms a daggerlike
structure. Lateral to this structure, on each side, are
triangular plates, and at their apices are the inner
spiracular teeth (1ST). The 1ST, saw, PAS, and the
SAd collectively represent the SAp (Fig. 2).
When the SAp is open, the 1ST and outer spi-
racular teeth (OST) are fully everted. The 1ST are
distal to the OST and are situated 1 set on either
side of the spiracular opening (Fig. 3a). These
structures are greatly reduced in comparison to the
OST, are dark in color, and are highly sclerotized.
The OST are lightly sclerotized and occur in 2
rows below and lateral to each set of 1ST Each of
the rows consists of 3 hooklike teeth that are
stacked one upon the other and when everted curve
backward and appear to be connected basally to the
PSL (Fig. 3a).
Fig. 1. Siphon and spiracular apparatus (closed) of Co-
quillettidia perturbans. (a) Lateral aspect. LSL, anterolat-
eral spiracular lobe; PSL, posterolateral spiracular lobe, (b)
Posterior aspect. S, siphon; PSP, posterior spiracular plate;
PSL, posterolateral spiracular lobe, (c) Anterior aspect.
SAW, saw; LSL, anterolateral spiracular lobe.
DISCUSSION
Most culicine larvae hang headfirst from the wa-
ter's surface, venturing below either to feed or es-
cape danger. Regardless of the time spent sub-
282
Journal of the American Mosquito Control Association
Vol. 1 8, No. 4
SAd
1ST
PAS
Fig. 2. Internal structure of the spiracular apparatus of Coquillettidia perturbans. SAd, spiracular apodeme; PAS,
postabdominal spiracles; SAW, saw; 1ST, inner spiracular teeth.
Fig. 3. (a) Spiracular apparatus (open) of Coquilletti-
dia perturbans. OST, outer spiracular teeth; 1ST, inner spi-
racular teeth; PSL, posterolateral spiracular lobe, (b) Cat-
tail (Typha latifolia) root cross-section exposing air-filled
aerenchyma channel pierced by spiracular apparatus of
Cq. perturbans.
merged, they must return to the surface to respire
through a siphon. However, exceptions exist and a
number of species found in permanent freshwater
habitats have evolved unique ways to circumvent
their need to surface for air, and perhaps reduce
their exposure to predation (McNeel 1 932, Van den
Assem 1958, Armstrong 1980). Some larvae rely
on trapped air bubbles in and among vegetation
whereas others remain submerged by utilizing gill
or cuticular respiration or both. Coquillettidia per-
turbans and its near relatives respire by piercing the
roots of emergent aquatic macrophytes with their
highly specialized siphons. Emergent aquatic mac-
rophytes are plants that are at least partly rooted in
sediment and whose leaves extend into the atmo-
sphere. The nutrient-rich medium in which these
plants grow is nearly anoxic, and because roots
need oxygen to function, some have evolved an
elaborate gas transport system. Dacey (1981) dem-
onstrated that aquatic plants such as the yellow wa-
ter lily (Nuphar luteum) maintain a pressurized
flow-through ventilation system in which atmo-
spheric air enters newly unfurled leaves against a
gradient in pressure and travels down the petioles
to the rhizomes and roots via a continuous network
of large open-channeled aerenchyma tissue. As a
result of the pressure generated by the younger
leaves, the by-products of root metabolism (carbon
dioxide and methane) are forced to the atmosphere
through the plants' older leaves. Compared with the
roots of terrestrial plants, those of emergent aquatic
plants contain a greater proportion of aerenchyma
(Armstrong 1978), and it is these larger air-filled
channels that larval Cq. perturbans utilize for un-
derwater respiration.
In New Jersey, 3 plant species in particular are
host to larval Cq. perturbans: cattail {Typha spp.),
rush (Juncus spp.), and swamp loosestrife (Deco-
don verticillatus) (Crans et al. 1986). Larvae
searching for an attachment site move along the
length of the root tapping the epidermis with the
apex of their siphon, occasionally everting the OST.
Ingram and Macfie (1917) proposed that fringed
bristles on the siphon of Mansonia africana (Theo-
bald) might have a sensory function that aids larvae
December 2002
Spiracular Apparatus of Coquillettidia
283
in the location of suitable attachment sites. Our ex-
amination of the siphon and SAp of Cq. perturbans
did not reveal any fringed bristles and in our opin-
ion larvae simply test the respiratory suitability of
the substrate by probing. The depth to which the
SAp is embedded in root tissue is dependent upon
its contact with suitable aerenchyma, and very of-
ten these channels can be found just inside the root
epidermis (Fig. 3b). Robust muscles attached to the
SAd provide the strength needed to pierce roots (In-
gram and Macfie 1917). Penetration of the epider-
mis is accomplished by rapid, repeated thrusts of
the terminal segments in concert with the exertion
of the daggerlike SAd and saw. Once a suitable
portion of the aerenchyma is penetrated, the 1ST
and OST open fully. In this position, the OST curve
backward and anchor the larva in place. This at-
tachment is a strong one, but larvae can quickly
release and swim away if disturbed. The 1ST seem
to play a lesser role in anchoring the larvae and
may represent a vestigial or ancestral character.
Larval Cq. perturbans have been assumed and
widely advocated to possess a siphon equipped
with an external saw that is used to cut into the
roots of aquatic plants. Our examination of the si-
phon and SAp reveals a much more complex struc-
ture composed of several internal structures that op-
erate in unison to 1st pierce and then anchor larvae
into the large air-filled aerenchyma channels found
in the roots of emergent aquatic macrophytes.
ACKNOWLEDGMENTS
We are grateful to Michael May and John La-
Polla, Department of Entomology, Rutgers Univer-
sity, for their assistance with the scanning electron
microscopy, and also James French, Department of
Plant Science, Rutgers University, for assistance
with the root dissection. This is New Jersey Agri-
cultural Experiment Station Publication D-08114-
14-01 supported by U.S. Hatch Act funds with par-
tial support from the New Jersey State Mosquito
Control Commission.
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